Transcript 14.pptx

COMSATS Institute of Information Technology
Virtual campus
Islamabad
Dr. Nasim Zafar
Electronics 1
EEE 231 – BS Electrical Engineering
Fall Semester – 2012
Bipolar Junction Transistors-BJTs
Lecture No: 14
Contents:
 Introduction
 Bipolar Transistor Currents
 Bipolar Transistor Characteristics and Parameter
 Early Effect
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References:
 Microelectronic Circuits:
Adel S. Sedra and Kenneth C. Smith.

Electronic Devices :
Thomas L. Floyd ( Prentice Hall ).

Integrated Electronics
Jacob Millman and Christos Halkias (McGraw-Hill).

Electronic Devices and Circuit Theory:
Robert Boylestad & Louis Nashelsky ( Prentice Hall ).

Introductory Electronic Devices and Circuits:
Robert T. Paynter.
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Reference:
Chapter 4 – Bipolar Junction Transistors:
Figures are redrawn (with some modifications) from
Electronic Devices
By
Thomas L. Floyd
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Bipolar Junction Transistors
BJTs-Circuits
C
B
E
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Transistor Types
 MOS - Metal Oxide Semiconductor
 FET - Field Effect Transistor
BJT - Bipolar Junction Transistor
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◄
Transistor Current Characteristics
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An Overview of Bipolar Transistors:
 While control in a FET is due to an electric field.
 Control in a bipolar transistor is generally considered to be due
to an electric current.
– current into one terminal
determines the current
between two others
– as with an FET, a
bipolar transistor
can be used as a
‘control device’
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Transistor Biasing Configurations:
1. Common-Base Configuration (CB) :
input = VEB & IE ; output = VCB & IC
2. Common-Emitter Configuration (CE):
input = VBE & IB ; output = VCE & IC
3. Common-Collector Configuration (CC):
input = VBC & IB ; output = VEC & IE
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Operation Modes:
 Active:
– Most importance mode, e.g. for amplifier operation.
– The region where current curves are practically flat.
 Saturation:
– Barrier potential of the junctions cancel each other out
causing a virtual short.
– Ideal transistor behaves like a closed switch.
 Cutoff:
– Current reduced to zero
– Ideal transistor behaves like an open switch.
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Operation Modes:
IC(mA)
Saturation Region
IB = 200 mA
30
Active Region
IB = 150 mA
22.5
IB = 100 mA
15
IB = 50 mA
7.5
Cutoff Region
IB = 0
0
VCE (V)
0
5
10
15
20
 Active: BJT acts like an amplifier (most common use).
 Saturation: BJT acts like a short circuit.
Cutoff: BJT acts like an open circuit.
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Common Emitter Characteristics:
 We consider DC behaviour and assume that we are
working in the normal linear amplifier regime with
the BE junction forward biased and the CB junction
reverse biased.
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Common-Emitter Output Characteristics
IC
Output Characteristic Curves - (Vc- Ic
Active
Region
IB
Region of Description
Operation
Active
Small base current
controls a large
collector current
VCE
Saturation Region
Cutoff Region
IB = 0
Saturation VCE(sat) ~ 0.2V,
VCE increases with IC
Cutoff
Achieved by reducing
IB to 0, Ideally, IC will
also equal 0.
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Common-Base-Configuration (CBC)
NPN Transistor
Circuit Diagram: NPN Transistor
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Common-Base Output Characteristics:
Although the Common-Base configuration is not the most common
configuration, it is often helpful in understanding the operation of BJT
Output Characteristic Curves - (Vc- Ic)
IC
mA
Breakdown Region
Saturation Region
6
0.8V
Active Region
IE
4
IE=2mA
2
IE=1mA
2V
4V
6V
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Cutoff
IE = 0
VCB
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Transistor Currents - Output characteristics:
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Common-Collector Output Characteristics:
Emitter-Current Curves
IE
Active
Region
IB
VCE
Saturation Region
Cutoff Region
IB = 0
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Bipolar Transistor Characteristics
21.4
• Behaviour can be described by the current
gain, hfe or by the transconductance, gm of the
device
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Conventional View & Current Components:
NPN Transistor-CEC
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Current Components:
NPN Transistor-CEC
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BJT Characteristics and Parameters
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BJT-Current Gain Parameters:
 Two quantities of great importance in the characterization of
transistors are the so-called common-base current gain ..
 and the so-called common-emitter gain .

DC  and DC 
 = Common-emitter current gain
 = Common-base current gain
Note:  and  are sometimes referred to as dc and dc
because the relationships being dealt within the BJT are DC.
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BJT-Current Gain Parameters:
 Common-base current gain  , is also referred to as hFB and
is defined by:
 = hFB = IC / IE
 Common-emitter current gain β, is also referred as hFE and
is defined by:
 = IC/IB
Thus:
IC  βIB
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Beta () or amplification factor:
 The ratio of dc collector current (IC) to the dc base current
(IB) is dc beta (dc ) which is dc current gain where IC and
IB are determined at a particular operating point, Q-point
(quiescent point).
 It’s define by the following equation:
30 < dc < 300  2N3904
 On data sheet, dc=hFE with h is derived from ac hybrid
equivalent circuit. FE are derived from forward-current
amplification and common-emitter configuration
respectively.
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 In the dc mode the level of IC and IE due to the majority
carriers are related by a quantity called alpha:
= IC
IE
IC = IE + ICBO
 It can then be summarize to IC = IE (ignore ICBO due to
small value)
 For a.c situations where the point of operation moves on the
characteristics curve, an a.c alpha defined by
  IC
IE
 Alpha a common base current gain factor that shows the
efficiency by calculating the current percent from current
flow from emitter to collector. The value of  is typical from
0.9 ~ 0.998.
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BJT-Current Gain Parameters:
 = Common-Base Current Gain (typical 0.99)
iC

iE
VBE
iC  I S e
iE 
IS

VT
VBE
e
VT
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BJT-Current Gain Parameters:
 = Common-emitter current gain (10-1000; typical 50-200)
iC

iB
VBE
iC  I S e
iB 
IS

VBE
e
VT
VT
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DC  and DC 
 = Common-base current gain (0.9-0.999; typical 0.99)
 = Common-emitter current gain (10-1000; typical 50-200)
 The relationship between the two parameters are:


 1


1
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Performance Parameters for PNP:
Common emitter dc current gain, dc:
I C   dc I B
But,
I C   dc I E   dc ( I C  I B )
  dc 
 I B
I C  
 1   dc 
 dc 
 dc
 T

1   dc
1   T
Note that  is large (e.g.  = 100)
For NPN transistor, similar analysis can be carried out. However,
the emitter current is mainly carried by electrons.
Example:
I EP

I EP  I EN
etc.
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Performance Parameters for PNP:
Emitter efficiency:
 
I EP
I
 EP
I EP  I EN
IE
Fraction of emitter current carried by holes.
We want  close to 1.
Base transport factor:
IC
αT 
I Ep
Fraction of holes collected by the collector.
We want T close to 1.
Common base dc current gain:
I C  T I EP  T I E  dc I E
 dc   T 
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Example: NPN Common-Base Configuration:
C
VCB
+
_
IC
B
Find:
IE ,  , and 
Solution:
IB
VBE
Given: IB = 50 m A , IC = 1 mA
+
_
IE
IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA
 = IC / IB = 1 mA / 0.05 mA = 20
E
 = IC / IE = 1 mA / 1.05 mA = 0.95238
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